Chapter12——声明抽象基类Shape，由它派生出3个派生类：Circle,Rectangle,Triangle，用一个printArea函数分别输出以上三者的面积，3个图形的数据在定义对象时给

声明抽象基类Shape，由它派生出3个派生类：Circle,Rectangle,Triangle，用一个printArea函数分别输出以上三者的面积，3个图形的数据在定义对象时给定。

#include<iostream>using namespace std;class Shape{public:virtual double area()=0;virtual void shapename()=0;};class Circle:public Shape{public:Circle(double r=1):radius(r){}virtual void shapename(){cout<<"Circle_area=";}virtual double area(){return 3.14159*radius*radius;}friend ostream&operator <<(ostream&,Circle&);protected:double radius;};ostream&operator <<(ostream&output,Circle&c){output<<c.area()<<endl;return output;}class Rectangle:public Shape{public:Rectangle(double a=1,double b=1):length(a),width(b){}virtual void shapename(){cout<<"Rectangle_area=";}virtual double area(){return length*width;}friend ostream&operator <<(ostream&,Rectangle&);protected:double length,width;};ostream&operator <<(ostream&output,Rectangle&re){output<<re.area()<<endl;return output;}class triangle:public Shape{public:triangle(double a=1,double b=1):base(a),height(b){}virtual void shapename(){cout<<"triangle_area=";}virtual double area(){return 0.5*base*height;}friend ostream&operator <<(ostream&,triangle&);protected:double base,height;};ostream&operator <<(ostream&output,triangle&tr){output<<tr.area()<<endl;return output;}void printArea(Shape **p,Circle a,Rectangle b, triangle c){p[0]=&a;p[1]=&b;p[2]=&c;for(int i=0;i<3 i="" p="" i="" -="">shapename();cout<<p i="" -="">area()<<endl;}}int main(){Shape *p[3];Circle c(10);Rectangle r(5,10);triangle t(3,4);printArea(p,c,r,t);return 0;}</p></3></iostream>